Radio receiver overload control circuit



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RECTIFIER H. RINIA ETAL Filed July 21, 1939 Al/c RADIO RECEIVER OVERLOADCQNTROL CIRCUIT CONVERTER A. F. AMPL.

RF. AMPL.

July 8, 1941.

uvvzzvroxs HERRE R/NIA, PIETER A. NEETESON, AND RgDUSAItTELLEEEN 7 -u-M/ATTORNEY.

SOURCEOF DIRECT cums/v7 FOR RECEIVER Patented July 8, 1941 RADIORECEIVER OVERLOAD CONTROL CIRCUIT Herre Rinia, Pieter A. Neetescn, andBernardus D. H. Tellegen, Eindhoven, Netherlands, as-, signors, by mesneassignments, to Radio Corporation of America, New York, N. Y., acorporation of Delaware Application July 21, 1939, Serial No. 285,700

1 Claim.

This invention relates to a circuit arrangement for avoiding overloadingof radio receiving sets.

According to the invention this is attained by including a rectifier inthe output circuit of a low frequency amplifying tube, preferably theoutput tube, said rectifier having such a threshold value that only withoverloading of the low frequency amplifier a rectified voltageisproduced in the output circuit of the rectifier, and which voltagecontrols the amplification of oneor more high, and/or intermediate,frequency amplification stages so as to reduce their amplification.

In a receiving set including automatic volume control in the high,and/or intermediate, frequency part the rectified voltage produced withoverloading in the output circuit of the rectifier is, according to afurther feature of the invention, preferably superimposed on the voltagethat serves to insure automatic volume control.

The invention will be more clearly understood by reference to theaccompanying drawing representing, by way of example, one embodimentthereof.

Low frequency oscillations are impressed on the grid circuit l2 of theoutput tube 3 of the low frequency amplifier of a radio receiving set,and, after having been amplified, supplied through an output transformer4 to a loudspeaker 5. A rectifying diode 6 is connected on the one handthrough a condenser 1, and on the other hand through resistance 8, 9, land H, to the primary winding of the output transformer. As appears fromthe drawing the resistances ill and H are connected to the source ofanode voltage l2 of the set. The resistance are so chosen that thevoltage across the resistance 10, which determines the bias of therectifying diode, has such a value that so long as overloading of theamplifier does not take place then the rectifier remains blocked. Assoon, however, as overloading occurs, and consequently the alternatingvoltage set up at the terminals of the primary winding of the outputtransformer exceeds the threshold value of the rectifier 6 determined bythe bias, there flows a rectified current. A voltage drop is producedacross the resistance 8, the value of this voltage drop depending on thedegree of overloading. This voltage drop is smoothed by means of aresistance I 3 and condenser l4, and is supplied through a conductor Ito the grid circuits of one or more high, and/or intermediate, frequencyamplification stages, and controls their amplification in a manner knownper so by grid voltage displacement, the amplification control GermanyMarch 14, 1939 taking place in such a manner that the amplification isreduced in the event of overloading.

In the represented circuit the voltage drop across resistance 8 issuperimposed on the usual control voltage for automatic volume control.Thi control voltage is set up across the resistance 9 and is obtained byrectification of the high, or intermediate, frequency oscillations in arectifier I6. In this circuit the voltage drop across the resistance 8will, in the event of overloading, decrease the amplification of thehigh, and/or intermediate, frequency amplifiers, so that the amplitudeof the oscillations supplied to the rectifier l6, and consequently thecontrol voltage across the resistance 9, will also decrease. Withoverloading the voltage drop across the resistance 8 will consequentlyincrease at first, whereas the control voltage across resistance 9decreases. Consequently the amplification of the high, and/orintermediate, frequency amplifier will at first vary to a slight extentonly. Further decrease of the amplification does not occur before thecontrol voltage across resistance 9 has been replaced by the voltagedrop across re-' sistance 8. In order that this takes place rapidly itis desirable to impress as high a possible a low frequency alternatingvoltage on the rectifier 6. However, this requires thorough smoothing ofthe voltage drop across the resistance 8, i. e., a large time constantof the resistance l3 and condenser This is favourable on the one hand inorder that upon the overmodulation ceasing to act the amplification ofthe high, and/or intermediate, frequency amplifier does not return toorapidly to its initial value, but on the other hand this large timeconstant may not affect the operation of the automatic volume control.Since, however, as appears from the drawing, the steadying condenser 14is connected to the junction of resistances 8 and 9 it is insured thatthe control is sufficiently slow with overloading, whereas the automaticvolume control takes place in a sufiiciently rapid manner.

The condenser I is preferably not too large in order that withoverloading there is not abruptly taken excessive energy from the outputcircuit of the output tube 3 whereby the reproduction might be affected.Suitable values for the individual circuit elements are:

Capacity 7=10,000 micromicrofarads Capacity 14:17:05 ,uf

Capacity 18:0.2 f

Resistors 8=13=0.8 m-ohm What is claimed is: y

In a radio receiver of the type provided with a radio signaltransmission network, a detector, a modulation voltage transmissionnetwork, means responsive to carrier amplitude variation for producing acontrol voltage proportional to carrier amplitude, at second meansresponsive to modulation voltage amplitude above a predeterminedoverloading value for producing a second control voltage proportional tomodulation voltage amplitude, said second means comprising a rectifiercoupled to said modulation voltage network, a load impedance in circuitwith the rectifier to develop said second control voltage, common meansapplying both said control voltages in a transmission efficiencyreducing sense to said radio transmission network, means connected tosaid load impedance for retarding the disappearance of the secondcontrol voltage with said modulation voltage decrease below saidoverloading value without affecting the development of said firstcontrol voltage, and said retarding means being a resistor-condensernetwork in shunt with said load impedance and having a relatively largetime constant.

HERRE RINIA.

PIETER A. NEETESON.

BERNARDUS D. H. TELLEGEN.

